Answer: It sounds like a stain from the Preen and I suspect it will wear off and disappear on its own eventually. Preen works to prevent weeds seeds from germinating by releasing a gas of sorts. The chemicals in Preen volitalize and evaporate into the air and what you are seeing is probably a residue from that action. At this point about all you can do is allow it to weather away on its own. If the rubber mulch is thick enough, you won't need to reapply a pre emergent product to keep weeds from sprouting. The thickness of the mulch should be sufficient to keep weed seeds from germinating. Best wishes with your landscape.
Q&A Library Searching Tips